The sampling frame for the Study is New York State (NYS), exclusive of New York City. The Study, which began September 4, 2007, is designed to use a population-based sampling strategy to recruit and compare the development of a matched-exposure cohort of infants, i.e., infants identified from pregnancies conceived with the help of infertility treatment, as noted in the NYS Perinatal Data System, and infants who were conceived without such therapies. The infertility treatment exposure is the basis for the sampling because of its relative rarity. For the primary cohort, infants identified as having been conceived by infertility treatment (exposed) were enrolled as participants at 3-5 months of age (gestation-corrected, if necessary) and followed longitudinally until age 36 months (3 years), along with a cohort unexposed infants, frequency-matched for residence within a NYS Regional Perinatal Network and plurality of birth. All co-twins of Study participants were enrolled simultaneously and either their outcomes noted (e.g., stillbirth, neonatal mortality) or they are being followed (secondary cohort) on the same schedule as Study participants, although the co-twins will not be considered as part of the primary cohort. Higher order multiples (triplets, quadruplets) were also enrolled and followed on the same schedule, regardless of exposure status. The recruitment window extended from September 17, 2008 (first mailing) through December 17, 2010 (final consent received), representing an approximate 2-year period of births. Recruitment goals were closely monitored so that the final recruited primary cohort sample was 1,297 exposed (infertility treatment) and 3,692 unexposed (no infertility treatment) mothers and infants, with twins making up about 22% of each cohort. There are 45 sets of higher order multiples (43 triplet, 2 quadruplets) recruited, such that there is a total of 5,034 mother/infant(s) sets. Parents of infants born in NYS are asked to participate in the Study for three years by: (a.) completing an initial questionnaire about the pregnancy, including, for example, confirmation of infertility treatment, time-to-pregnancy, infant feeding, and other demographic and socioeconomic factors (i.e., maternal education, occupation) and confounders associated with motor and social development; (b.) completing a standardized screening instrument that addresses their childs motor and social development at chronologic or gestation-corrected ages 4, 8, 12, 16, 20, 24, 30, and 36 months; and (c.) providing information on the child's growth and medical history (e.g., illnesses, diagnoses of birth defects, major impairments). Information on growth and medical history are assembled in journal format from physicians examination data. Records will be linked to other pertinent health records (e.g., early intervention programs, WIC, birth defect or cancer registries). Several additional components have been added: (a.) a detailed ART history by linking with the Society for Assisted Reproductive Technology (SART) cycle-based database; (b.) the acquisition of residual blood specimens left over from mandatory neonatal screening and laboratory testing for biomarkers and environmental exposures. The blood specimens are owned and archived at the Wadsworth Center Laboratories (New York State Department of Health), and (c.) plans for the administration of a standardized assessment for diagnosing developmental disabilities using a battery of empirically validated and reliable instruments around 36 months of age for all infants and children who fail any of the neurodevelopmental screens. We found the NYS live birth certificate registry to be a suitable sampling framework for work focusing on ART and children's health, given that birth certificate's capture of ART treatment was high relative to maternal report. Data analysis is underway for key research questions The baseline study design and methods paper has been published to aid other investigators in the design and conduct of similar research.